Communication devices such as UEs are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two UEs, between a UE and a regular telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
UEs may further be referred to as wireless terminals, mobile terminals and/or mobile stations, mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is being served by a network node. A cell is the geographical area where radio coverage is provided by the network node.
The network node may further control several transmission points, e.g. having Radio Units (RRUs). A cell can thus comprise one or more network nodes each controlling one or more transmission/reception points. A transmission point, also referred to as a transmission/reception point, is an entity that transmits and/or receives radio signals. The entity has a position in space, e.g. an antenna. A network node is an entity that controls one or more transmission points. The network node may e.g. be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size.
Further, each network node may support one or several communication technologies. The network nodes communicate over the air interface operating on radio frequencies with the UEs within range of the network node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the UE to the base station.
Long Term Evolution (LTE) is a radio access technology standardized by the 3rd Generation Partnership Project (3GPP). In LTE, network nodes, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks. In LTE the cellular communication network is also referred to as E-UTRAN. The standard is based on Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Single Carrier—Frequency Division Multiple Access (SC-FDMA) in the uplink.
An E-UTRAN cell is defined by certain signals which are broadcasted from the network node. These signals contain information about the cell which can be used by UEs in order to connect to the network through the cell. The signals comprise reference and synchronization signals which the UE uses to find frame timing and physical cell identification as well as system information which comprises parameters relevant for the whole cell.
In the time domain in the downlink, one subframe is divided into a number of OFDM symbols. One OFDM symbol further comprises a number of sub-carriers in the frequency domain. One OFDM symbol on one sub-carrier is referred to as a Resource Element (RE), shown as squares in FIG. 2 below.
In LTE, no dedicated data channels are used, instead shared channel resources are used in both DL and UL. These shared resources, are each controlled by a scheduler that assigns different parts of the DL and UL shared channels to different UEs for reception and transmission respectively.
The assignment information for where to find the payload data on the shared channels are transmitted in a control region covering a few OFDM symbols in the beginning of each downlink subframe. The data is transmitted in a data region covering the rest of the OFDM symbols in each downlink subframe. The size of the control region is either, one, two, three or four OFDM symbols and is set per subframe. The size is signaled as a specific Control Format Indicator (CFI) to the UE for each subframe on the so called Physical Control Format Indicator Channel (PCFICH). In order to allow data in form of voice services as well as data services to be sent over LTE, high data transfer rates are necessary. This is achieved by keeping the size of the control region to a minimum at all times, thereby maximizing the size of the data region.
Each assignment, i.e. a pointer to the set of REs where the payload data is actually sent, is transmitted on a physical channel named Physical Downlink Control Channel (PDCCH) in the control region. There are typically multiple PDCCHs in each subframe and the UEs will be required to monitor the PDCCHs to be able to detect the assignments directed to them and in that way being able to find the data directed to them.
The PDCCH is mapped to a number of Control Channel Elements (CCEs), each CCE comprising a set of 36 REs. The CCEs may be scheduled in the subframe as an aggregation of 1, 2, 4 or 8 CCEs. These four different alternatives are herein referred to as aggregation level 1, 2, 4, and 8 respectively. The variable size achieved by the different aggregation levels is used to adapt the coding rate to a required Block Error Rate (BLER) level for each PDCCH. The total number of available CCEs in a subframe will vary depending on, among other things, the number of OFDM symbols used for control. The CCEs which make up the PDCCH, will be spread in time and frequency in a pseudo random manner within the control region. A few of the REs within the control region will however be used for Cell specific Reference Signals (CRS). These REs will not be used by CCEs, at least not within the same cell.
The CRS are UE known symbols that are inserted in a RE of the subframe of an OFDM time and frequency grid and broadcasted by the network node. The CRS are used by the UE for downlink channel estimation. Channel estimation is used for demodulation of downlink data both when the UE is in connected state and is receiving user data and when the UE is in idle state and is reading system information. Due to the latter use case, the CRS must be transmitted even from cells which do not have any UEs connected.
In case several antennas are used by the network node for transmitting and each antenna is representing a cell, each antenna has to transmit a unique CRS in order for the UE to connect to that specific cell. When one antenna transmits, the other antennas have to be silent in order not to interfere with the first antennas CRS. To reduce the interference of reference signals between the cells, the position in the grid of the CRS is usually shifted in frequency between the cells by applying a frequency offset between the CRS sent in the different cells on a site.
The main advantages using such configuration are low interference on CRS and low level of contamination of channel quality estimates. On the other hand, shifted CRS will introduce increased interference on channels used for signaling as well as data in neighboring cells. Although this solution reduces the interference of CRS between cells, it has the problem that the CRS of one cell will disturb the PDCCH and the Physical Downlink Shared Channel (PDSCH) symbols of neighboring cells.
This has shown to affect handover performance negatively in a significant way, since the assignment to a handover command via the PDCCH is affected by the interference from the shifted CRS in neighboring cells. The reduction of handover performance often leads to loss of service for the affected UE. This is especially a problem when voice services are provided, since a dropped call due to loss of service will not be accepted by a user.